The present invention is directed to well strainers or screens, and more particularly concerns a well strainer composed of a plurality of annular filtering elements stacked on top of one another so as to form a cylindrical filtering structure and spaced from one another for use in filtering fluids such as water, oil, and gas, which are extracted from bores or wells.
Typical well strainers or screens are made up of a plurality of annular filter sections or rings stacked one on top of the other and joined together by long threaded rods extending the length of the stack to form a cylindrical strainer. As described, for example, in U.S. Pat. No. 1,705,848, a well screen includes a plurality of ring like concrete sections arranged in superimposed relation and spaced from one another by having each section provided with a series of depending lugs which rest upon the upper surface of the adjoining section, and thereby define a preset, non-adjustable, gap between sections. Similarly, U.S. Pat. No. 3,822,744 discloses a straining tube composed of a plurality of superimposed rings with each ring having circumferentially arranged spacers and supports which determine a preset, non-adjustable, gap between adjacent rings and provide stability to the structure.
In order to accommodate different well bore conditions (soil, clay, or rock), to filter out different sizes of solid particles, and to extract different fluids, it is desirable to be able to adjust the gap between adjacent rings during construction of the well strainer so that one need not stock a variety of different well strainers or screens to suit various conditions. For example, U.S. Pat. No. 4,752,394 discloses a bore screen which is constructed from stacked rings with the space between the rings being adjustable. Each ring is formed on opposite first and second faces with aligned circumferentially spaced bearing pads. The bearing pads on the first face of successive rings contact those on the second face of adjacent rings and serve to space the rings apart. The bearing pads on the first face of each ring are formed with stepped projections and those on the second face of each ring are formed with complimentary stepped recesses. The stepped projections and stepped recesses associated with respective ones of the bearing pads on each ring are configured differently from the step projections and step recesses associated with others of the bearing pads whereby the spacing between adjacent rings is selected during bore screen assembly by rotating one ring relative to another to bring different pairs of bearing pads into contact with one another. In the specific example shown in the patent, the rings may be stacked and clamped together to form a bore screen having either 0.50, 0.75, or 1.0 mm gap between rings. Each of the bearing pads on the rings has a cylindrical central opening for receiving one of the threaded studs which extend through the stack of rings. In order to provide for three positions of adjustability, each ring has six bearing pads.
Although the bore screen of above-described U.S. Pat. No. 4,752,394 provides for a limited amount of adjustability in the gap between rings and, as such, is an improvement over well screens which do not provide for adjustability, this bore screen arrangement suffers from two major drawbacks. First, the gap between rings must be selected prior to assembly of the screen, and cannot be changed without completely disassembling the screen, that is, removing all of the threaded studs and rotating each ring relative to the other. Second, the large number of bearing pads tends to obstruct the flow of fluid between the rings by reducing the effective filtering area.
Hence, there is a need for an improved well strainer which not only allows for adjustment of the gap between adjacent stacked filter segments prior to assembling the stack but also allows for adjustment of the gap between filter segments after assembly and does so without requiring complete disassembly and which does not unduly reduce the effective filtering area of the strainer.